The research proposed is aimed at characterizing quantitatively the membrane interactions and potential-sensing mechanisms of certain classes of voltage-sensing dyes that are particularly promising tools for monitoring membrane potential changes in heart and nerve cells. Experiments are proposed for determining whether and how these dyes perturb membrane permeability. These studies are based upon the effects of these dyes in altering the ion permeability of lipid bilayer and cell membranes. In addition, spectroscopic studies are proposed, in bilayer and cell membrane systems, for characterizing how variations in membrane composition and aqueous ionic strength alter the spectral responses of these dyes to a change in membrane potential. Preliminary results are presented which indicate that the differences which are observed in different species for dye spectral responses to a transmembrane voltage can be surface potential differences among different species. In addition, a novel method for using certain types of dyes as specific "probes" of the membrane's surface potential is proposed along with preliminary results indicating the feasibility of this method. The development of this technique along with the other studies proposed is expected to provide a simple method for "calibrating" the spectral responses of these dyes so that they can be used as "probes" of the absolute values of transmembrane potentials in any type of cell.